Known overcurrent trip and installation switching devices of the kind mentioned can be electro mechanical devices. The point of contact can include a fixed contact member and a movable contact member which is held by a movable contact arm or contact bridge. In the closed position the movable contact member is pressed against the fixed contact member influenced by the force of a contact spring.
Known trip devices and installation switching devices can also include a mechanical gear mechanism with a latch and a spring force based energy storage assembly.
Further a known tripping device in the event of a tripping condition acts on the latch, which then releases the energy from the energy storage so that the gear mechanism can act upon the contact lever or contact bridge in order to open the point of contact.
A tripping action of the overcurrent tripping device can be triggered if the current flowing through the installation switching device exceeds the nominal current considerably over a given period of time. The time that has to pass by until a tripping event occurs depends on the strength of the overcurrent. The stronger the overcurrent, the shorter the time until a tripping action occurs. The characteristic dependence between overcurrent and trip time is called the “time invert trip curve”. There are standards describing the time invert trip curves, classified in so called trip classes. At an overcurrent which is e.g. 1.5 times the nominal current, for example, can have trip times are between 1 and 10 minutes, for overcurrents 3 times higher than the nominal current trip times are in the range of 2 to 40 seconds, and for overcurrents in the range of 1.1 times the nominal current trip times can be as long as 30 minutes to several hours.
Known overcurrent tripping devices can use metal strips made of a bimetal or a thermal shape memory alloy as an actuating member. The bimetal strip can be heated up by the current flowing, either directly or indirectly, and heating causes the bimetal strip to bend. The thermal properties of the bimetal strip can be designed such that in case of nominal current the bending of the bimetal strip is small enough so that no tripping action occurs. If however an overcurrent flows for some time, the bending becomes large enough to cause an interaction of the bimetal strip, either directly or indirectly via a tripping lever, with the gear mechanism which then causes the contact point to open. Such a device is shown for example in DE 10 2005 020 215 A1.
Such known thermal overcurrent tripping devices suffer from a cross-influence to ambient temperature. Increasing ambient temperature can cause a bending that adds to the current-induced bending and would reduce the tripping threshold if not compensated for. Known solutions for compensating the ambient temperature effect can be based on the application of a second bimetal strip, called compensation bimetal, which is not heated by the current flow but only due to ambient temperature and whose bending direction is opposed to that of the tripping bimetal. The temperature range that can be compensated by such compensation bimetals is however limited.
There are applications where circuit breakers are to be applied in an environment where a high ambient temperature variation might occur, for example up to 70° C., and where the cross-sensitivity of the tripping threshold should be minimal. There are no compensation means known to allow the reliable application of an installation switching device like a circuit breaker in applications with such large variations of ambient temperature.